code: def test_mutable_state(self):
      inputs = Input(shape=(3,), batch_size=2, name="input")
      x = layers.Dense(5)(inputs)
      outputs = layers.Dense(5)(x)
      model = Functional(inputs, outputs)
      # Allow attaching state to a model that isn't directly part of the DAG.
      # Most useful for functional subclasses.
      model.extra_layer = layers.Dense(5)
  
imports: import warnings
  import numpy
  import pytest
  from keras import backend
  from keras import layers
  from keras import testing
  from keras.layers.core.input_layer import Input
  from keras.layers.input_spec import InputSpec
  from keras.models import Functional
  from keras.models import Model
label_desc: Use image node_id 3 for calling the FunctionalTest obj's underlying member method code with example usage: obj.test_mutable_state() without return types